Green electricity is electriciy generated using "green" methods, eg wind, solar, etc. When a person swiches to a 'green' electricity tariff, s/he would be making an arrangement with the supplier that the electricty the university consumes will be supplied for by green methods, and the university's electricity bill will go towards investment in green energy supplies.

That's all very cool, but what if there was a considerable demand for green electricity, and everone on green tarrifs consumed more electricity than can be generated by green methods currently in existance? That would mean that they will inevitably have at least some of the electricity generated by "brown" methods (coal, oil, gas, nuclear, rubbish inceneration...)

The problem is that it's uncertain if the green electricity companies will be able to or even want to keep their promises, and so just switching to green electricity in itself will not be the ultimate solution. Pressure needs to put on universties (and ordinary consumers) to reduce energy consumption, and also on the electricity generation companies, so that they will go ahead and build more green power generation methods to meet the demand. It we do make the switch and but then just rest on our laurels, we will potentially allow the compaines supplying the green electricity to carry on supplying brown electricity to people people requesting green electricity, but our ulitmate goal as environmental campaginers should be to eliminate brown electricity and ensure that all electricity is green electricity.